Sometime later, in my experimentation with turning my broken cache
   back on in the presence of putative cache-flushing code, my system
   panic'ed with a "dup alloc" message.  Could it be that the mild
   curdling that was left over from the previous reboot caused that,
   rather than my cache scrambling DMA?  (There was some other damage on
   the filesystems when I rebooted, but that might have been from the
   sudeen halt.)

i saw this panic yesterday with a recent kernel i had built (i
didn't pay much attention -- i was hacking elsewhere).  i didn't
think much of it at the time, but i do now think you're right.

.mrg.